The invention relates to a grounding electrical adapter for coupling a grounding blade of an electrical plug to a grounded screw of an electrical receptacle, for example when couplin a three-blade grounding plug to a two-slot coupling of a two-wire receptacle.
Much of the present day electrical equipment uses three-wire cords having three-wire plugs adapted for plugging into three-wire receptacles. Two of the wires are the powerline wires (a hot and neutral wire) and the third wire is a grounding wire typically connected to any exposed metal portions that the equipment may have. One purpose of the grounding wire is to maintain such exposed portions grounded and thus reduce hazard in the eventuality that an exposed electrically conductive portion becomes connected to the hot side of the powerline. While many receptacles are of the three-wire type, there are some, primarily in older buildings, which are of the two-wire type and do not have a grounding female terminal for receiving the grounding blade of a three-wire plug. It is possible to connect three-wire equipment to such two-wire receptacles by simply cutting off the grounding blade of the three-wire plug, or by replacing the three-wire plug with a two-wire plug, but this is obviously unsafe because it fails to make use of the safety afforded by the grounding wire.
The two-wire receptacles typically have a metal housing which is electrically grounded (through a metal conduit enclosing the two powerline wires, or through a separate, grounding conductor), and have a cover plate secured to that grounded housing by a metal screw. Based on this, there have been prior art adapters designed to electrically couple the grounding blade of a three-wire plug to that grounded screw. One commonly used adapter of this type has a back end where a three-blade plug can be plugged in, a front end having two blades for plugging into the outlet socket of a two-wire receptacle, and a wire coming off the side of the adapter and having at its free end a crimp terminal which can be connected to the grounded, plate-mounting screw of the receptacle by loosening or removing the screw, placing the crimp terminal under the screw and then tightening the screw to make electrical contact with the crimp terminal and to keep the crimp terminal in place against the cover plate of the receptacle. Other adapters of this type have a stiff metal strip coming off the side. The metal strip is similarly connected to the grounded screw of the receptacle, requiring however that the screw be completely removed and passed through an opening in the strip (U.S. Pat. No. 3,865,453).
There are additionally prior art techniques where the free end of a three-wire cord has a two-blade plug for plugging into a two-wire receptacle and a grounding wire comes off the side of the plug to be connected to the grounded screw of the receptacle (U.S. Pat. No. 3,072,873). In a similar prior art technique, the grounding wire of the equipment is connected to a metal pin coming off the side of a two-blade plug, and -- as the plug is being plugged into the two-wire receptacle -- the pin engages a metal spring-clip previously attached to the receptacle by the grounded screw (U.S. Pat. No. 3,349,360). There are also devices in which a three-blade plug is inserted into a three-wire receptacle and the grounding wire of the cord is additionally connected, for extra mechanical and electrical safety, to the grounded screw of the receptacle by a metal strip coming off the side of the plug (U.S. Pat. No. 3,381,258), as well as devices in which the neutral wire of a two-wire plug is additionally connected to the grounding female terminal of a three-wire receptacle, or to the grounded screw of a two-wire receptacle (U.S. Pat. No. 3,739,317).
There are various disadvantages with the above types of prior art adapters for coupling the grounding blade of a plug to the grounded screw of a receptacle, and particularly with the adapters for coupling a three-blade plug to a two-wire receptacle. For example, the grounding wire coming off the side of the common adapter is often not connected to the grounded screw of the receptacle because the user does not appreciate its significance, or because it is inconvenient at the time to connect it properly, or for other reasons. As another example, it may be inconvenient or dangerous to use adapters of the type which have a fixed metal strip coming off the side, because such adapters typically require that the grounded screw be completely taken out of the receptacle, which may displace the plate. The hazard of this is that the plate may make electrical contact with one or both blades of the plug or adapter which are partly plugged into the receptacle and at a dangerous potential.
This invention obviates problems of the type discussed above by providing an adapter which not only assures good electrical and mechanical contact to the receptacle, but also tends to force a user to make a convenient and proper ground connection. Moreover, the grounded screw of the receptacle plate does not have to be removed, but only has to be slightly loosened, thus completely obviating the electrical hazard and the inconvenience of having the receptacle plate come off its proper place.
The invented adapter has a grounding assembly moveable between an inoperative position and an operative position. When the grounding assembly is in its operative position, a grounding terminal of the assembly is aligned with the grounded screw of a receptacle into which the adapter is plugged, and a cord plug having a grounding blade may be plugged into the back end of the adapter. When the grounding assembly of the adapter is in its inoperative position, the grounding terminal of the assembly is offset from the grounded screw of a receptacle into which the adapter is plugged, and a back portion of the grounding assembly blocks the grounding blade of a cord plug from being plugged into the adapter. If an attempt is made to force the grounding blade of a plug into the adapter while the grounding assembly is in its inoperative position, the grounding assembly moves from its inoperative to its operative position, due to a cam action between the leading end of the grounding blade being inserted into the adapter and a suitable cam surface at the back end of the grounding assembly, so as to automatically align the grounding terminal of the adapter with the grounded screw of a receptacle where the adapter is plugged in.